Accelerating Cavities for an 800 MeV Soc

The minimum required energy gain per turn for the 800-MeV Separated Orbit Cyclotron increases by a factor of about five from the injection radius to maximum orbit radius. Ordinary rectangular cavities operating in the TM11O mode can be used to provide the accelerating voltage; however, the cavity length is then about twice the distance between the inner and outer orbits (~18 ft). This length, and the rf power loss, can be reduced by shaping the cavity to excite in addition the TM210 and TM310 modes. Inclusion of these higher order modes shifts the maximum voltage from the midpoint of the cavity out toward the end, resulting in a shorter cavity and lower losses. The performance of a 1/4-scale model of the "shaped rectangular cavity" was found to agree quite well with theory. "Wedge-shaped" cavities were also investigated. In this cavity, which is a sector of a cylinder, the length of the accelerating gap increases with machine radius. The upper and lower boundaries of either type of cavity can be shaped to excite higher order Mlodes.